Optimize get_top_traders to only fetch requested candidate set

[snowrugar-beep]

Summary

Optimized get_top_traders to only fetch the requested candidate set instead of always fetching 100 entries.

Changes

• peerx-contracts/counter/src/lib.rs: get_top_traders now fetches max(limit, 100) candidates instead of hardcoded 100; apply_trader_limit no longer caps at 100 (caller controls this)
• peerx-contracts/counter/portfolio.rs: Removed hard cap of 100 from get_top_traders — the function now iterates exactly up to the requested limit (O(limit) instead of O(100))
• Net reduction of 9 lines
• No unrelated changes introduced

Closes #69

[ameeribro4-sudo]

LGTM